The current trend by a growing number of gallium arsenide (GaAs) chip makers is to supply low cost, high performance analog and digital integrated circuits (ICs) capable of operating in the 1-60 gigahertz (GHz) frequency range. To take full advantage of this new and exciting semiconductor technology, there is a desperate need for packaging and packaging mass production assembly techniques which will permit system designers to efficiently translate GaAs device performance to a systems level.
Early microwave chip carriers (MCCs) and monolithic microwave integrated circuit (MMICs) packages were fashioned after the ceramic packages utilized by the silicon technology. Unfortunately, these hermetically sealed enclosures exhibited poor impedance matching, high insertion loss and low RF isolation, as well as seal ring and cavity resonance during microwave operations. As a result, system designers were forced to seek alternative packaging approaches. As an outline, the interested reader is encouraged to review the suggestions highlighted in the following articles:
1.) D. E. Heckman, et al. "Microwave Chip Carrier for Monolithic Integrated Circuits," IEEE GaAs IC Symposium, pp 155-158, 1985;
2.) B. A. Ziegner, "High Performance MMIC Hermetic Packaging," Microwave Journal, vol.29, no.11., pp. 133-139, November 1986;
3.) R. S. Pengelly and P. Schmacher, "High Performance 20 GHz Package for GaAs MMICs," Microwave systems News, vol.18, no.1, pp. 10-19, January 1988;
4.) J. A. Frisco, D. A. Haskins, D. E. Heckman, D. A. Larson, "Low Cost T.O. Packages for High Speed/Microwave Applications", in 1986 IEEE MTT-S Int. Microwave Symposium Digest, June 1986, pp. 437-440; and
5.) J. Cook, "System Level Considerations for Microwave/Millimeter Wave Packaging", GaAs IC Packaging Short Course, IEEE GaAs IC Symposium, Nov. 6, 1988.
The work accomplished by these microwave design teams has given the industry a number of techniques by which MMIC packages can be designed. Unfortunately, these packages comprise expensive custom assemblies which are unsuitable for mass-production or wide scale industry integration.
In most cases, ceramic still comprises the primary substrate material for the MMIC package due to its dielectric properties, thermal expansion properties and ability to host thin film (or thick film) hybrid circuitry. These properties aid in reducing transmission line losses and allow line widths to be readily varied. These devices are typically characterized by a machined metal cavity and an associated metal coaxial cable for providing a hermetically sealed enclosure.
Notwithstanding, RF isolation remains a critical design problem with ceramic GaAs MMIC packages. Controlling RF isolation is a matter of controlling the mechanisms by which a signal on one path is coupled to another path in the package. This problem is accentuated when signal frequencies of 12 GHz or higher are routed along parallel line patterns. While it is appreciated that the ideal solution requires placing a conductive wall between each signal path, ceramic's inability to be readily molded for customized designs and its high cost serve as barriers to the realization of a commercially mass-producible microwave device package. From the foregoing discussion it should be evident that existing microwave device carriers suffer in economy and performance due to the types of materials used (primarily ceramic), the type of transitions needed, and the number of piece parts needed to form a hermetic package.
Recent developments in the Polymer technology have resulted in the use of plastics as circuit carriers for sub-microwave frequency applications. Molded "three dimensional" printed circuits are known only for low frequency applications. Printed circuits are considered "three dimensional" when the substrate comprises a surface variation in at least one dimension.
These molded circuit carriers are plastic and are formed by injection molding, extrusion or other conventional thermoplastic fabrication processes. Therefore, they may be shaped to avoid or adapt to physical constraints in a specific application. Patterns of conductors are metallized to predetermined surfaces of the substrate and serve as circuit traces and bonding paths for circuit components. Once components are fixed, it becomes a multi-device carrier.
Plastic is generally characterized as a high loss, non-hermetic material and has not traditionally found use as a material in the microwave carrier design. In addition to providing poor RF isolation, plastic typically possesses poor electrostatic dissipation (ESD) properties. Notwithstanding, it would be extremely advantageous to provide a high frequency electrostatic RF attenuating circuit carrier assembly which overcomes the above cited short comings.